The present invention, in some embodiments thereof, relates to load balancing and or interference minimization in wireless networks and, more particularly, but not exclusively, to such load balancing etc. in a WiMAX or like wireless cellular network, in particular including any networks using the OFDMA protocol.
WiMAX, or Worldwide Interoperability for Microwave Access, is a telecommunications technology that provides wireless data in a variety of ways, ranging from point-to-point links to full mobile cellular-type access. The name “WiMAX” was created by the WiMAX Forum, which was formed in June 2001 to promote conformity and interoperability of the standard. The forum describes WiMAX as “a standards-based technology enabling the delivery of last mile wireless broadband access as an alternative to cable and DSL”.
The bandwidth and range of WiMAX make it suitable for potential applications including: connecting Wi-Fi hotspots with other parts of the Internet, providing a wireless alternative to cable and DSL for last mile broadband access, providing portable connectivity, to allow users to connect through portable devices without restriction to available hardware, providing data and telecommunications services, and providing a redundant source of Internet connectivity to protect from service loss at the main connection.
In Wi-Fi the media access controller (MAC) uses contention access—all subscriber stations that wish to pass data through a wireless access point (AP) compete for the AP's attention on a random interrupt basis. This can cause subscriber stations distant from the AP to be repeatedly interrupted by closer stations, greatly reducing their throughput. This makes services such as Voice over IP (VoIP) or IPTV, which depend on an essentially-constant Quality of Service (QoS) depending on data rate and interruptibility, difficult to maintain for more than a few simultaneous users.
In contrast, the 802.16 MAC uses a scheduling algorithm for which a mobile station needs to compete only once for initial entry into the network, hereinafter referred to as network entry or NWE. After NWE the mobile station is allocated an access slot by a base station. The time slot can enlarge and contract, but remains assigned to the subscriber or mobile station, which means that other subscribers cannot use it. In addition to being stable under overload and over-subscription (unlike 802.11), the 802.16 scheduling algorithm can also be more bandwidth efficient. The scheduling algorithm also allows the base station to control QoS parameters by balancing the time-slot assignments among the application needs of the subscriber stations.
The current IEEE-802.16e standard in fact allows the mobile station (MS) to select the most appropriate base station (BS) from those available. The standard however does not define the BS selection method that the MS is to use. The BS selection methods for network entry (NWE) and MS handover (HO) that are currently used are vendor specific.
The MS has available the following two factors:                Channel conditions        BS load        
Channel condition is set according to measurements performed by the MS using known metrics such as received signal strength indication (RSSI), signal to interference noise ratio (SINR), etc. A particular feature of the known channel selection method is that it is transparent to the individual user so that two mobile stations requiring the same quality of service have the same chance of receiving attention at the same base station.
The mobile station knows the base station load because the Base Station Load is broadcast by BSs in downlink channel descriptor (DCD) messages and uplink channel descriptor (UCD) messages, messages which are sent periodically by the base station to define downlink and uplink parameters and in neighborhood advertisement (NBR-ADV) messages, messages sent periodically by the base station to define the downlink and uplink parameters of its neighbors. Parameters that may be obtained from such messages include Available downlink (DL) Radio Resources and Available uplink (UL) Radio Resources, and these parameters indicate the average percentage of unused slots in the DL and UL.
The MS can use downlink and/or uplink resource parameters or just one of them for the NWE or HO decision, but the MS currently has no means to combine the two parameters into a single decision function that considers both the downlink and uplink. Moreover, the MS is unable to introduce its QoS requirements, i.e., current connections' requirements, (which also impact the required slots) in any except the crudest way, meaning that it simply selects the BS with the most spare slots. However choosing the base station with the most available slots is not always the optimal choice, say considering interference and other issues. It is not currently possible to make a choice that optimizes for a low QoS requirement.
In addition, the MS is unable to take into consideration the interference it imposes on the network upon BS selection. Thus a MS may be more distant from a first BS than a second BS, but if the first BS has more slots then it may choose the first BS in preference to the second. However because of the increased distance it may require a stronger signal, thus interfering with numerous other MSs, in some cases unnecessarily.
Interference thus causes difficulties due to the fact that the consumption of slots (and hence the load on the BS) is defined according to the channel's conditions and modulation and coding scheme (MCS). A single MS with poor channel conditions may have an affect on the BS's load that is out of all proportion to its actual requirements. In addition, the higher the QoS requirements are, the higher the MS consumption and loading affect. Furthermore, when an MS selects a more distant BS it affects the neighboring BSs and more particularly degrades the performance of the neighboring base stations due to interference.
WO02/32156 to Nortel teaches network driven cell switching and handoff with load balancing for wireless systems. The system uses two metrics—signal strength and base station load. A function takes these two metrics as inputs and produces a factor. The higher the factor the better is the choice of base station. However, this does not take into account the mobile station's actual consumption, or effect on the actual base station load.
To conclude, currently the IEEE-802.16e standard does not standardize the base station selection method and the various vendor-specific methods in use do not include a way to combine all factors influencing the consumption of slots into a single BS selection method/decision. Rather they are transparent to the actual users so that two mobile stations requesting the same quality of service stand the same chance at the same base station. At most the base station is able to modify connections after the mobile station has connected.